Investigations of the earth's atmosphere have shown that solid aerosol particles are distributed throughout the atmosphere in varying concentrations. In general, the concentration of any particular species of aerosol particles in the earth's atmosphere at any particular geographical location and altitude varies primarily with seasonal changes. A discussion of the origin and distribution of aerosol particles in the earth's atmosphere is provided in an article by J. M. Prospero et al. entitled "The Atmospheric Aerosol System: An Overview" published in Reviews of Geophysics and Space Physics, Vol. 21, No. 7, pages 1608-1629, (August 1983). Other publications discussing the phenomenology of atmospheric aerosol particles include:
(1) "Stratospheric Aerosol Measurements I: Time Variations at Northern Latitudes" by D. J. Hofmann et al., Journal of the Atmospheric Sciences. Vol. 32, pages 1446-1456, (July 1975); PA1 (2) "Stratospheric Aerosol Measurements II: The Worldwide Distribution" by J. M. Rosen et al., Journal of the Atmospheric Sciences, Vol. 32, pages 1457-1462, (July 1975); PA1 (3) "Effect of the Eruption of El Chichon on Stratospheric Aerosol Size and Composition" by V. R. Oberbeck et al., Geophysical Research Letters, Vol. 10, No. 11, pages 1021-1024, (November 1983); PA1 (4) "Satellite and Correlative Measurements of the Stratospheric Aerosol III: Comparison of Measurements by SAM II, SAGE, Dustsondes, Filters, Impactors and Lidar" by P. B. Russell et al., Journal of Atmospheric Sciences, Vol. 41, No. 11, pages 1791-1800, (June 1984).
Techniques were developed in the prior art that make use of atmospheric aerosol particles for obtaining various kinds of air data measurements. For example, B. M. Watrasiewicz and M. J. Rudd showed in Laser Doppler Measurements. Butterworth & Co. (Publishers) Ltd., Section 3.8.4 at pages 67-68, (1976), that an anemometric velocity measurement can be obtained by measuring the intensity of light scattered in three different directions by a moving particle passing through a focal volume. Photodetectors fixedly positioned at three different locations with respect to the focal volume collect light scattered by the moving particle, and generate three corresponding electrical signals proportional to Doppler shifts between the light scattered from the laser beam and the light scattered from the fixed scatterer in three different directions. From these Doppler shifts in three different directions, three corresponding components of the velocity of the moving particle relative to the photodetectors can be calculated. From these three velocity components, a vector velocity measurement for the moving particle can be obtained.
In U.S. Pat. No. 4,506,979, the principle of laser Doppler velocimetry was applied by P. L. Rogers to the measurement of the velocity of an aircraft relative to the motion of aerosol particles in the surrounding atmosphere. According to the technique described by Rogers, three pairs of laser beams derived from a common source are focussed at a focal volume located at a predetermined distance from the aircraft. The two laser beams comprising each pair are coherent, and therefore interfere with each other to produce a three-dimensional fringe plane pattern. Three sets of three-dimensional fringe plane patterns are thereby obtained, corresponding to the three pairs of interfering laser beams focussed at the focal volume. A non-orthogonal three-dimensional coordinate system is defined by three unit vectors oriented along the respective directions of propagation of the corresponding three pairs of interfering laser beams. Each set of fringe plane patterns moves in a direction perpendicular to its corresponding unit vector. The motions of the three different sets of fringe plane patterns produced as an aerosol particle passes through the focal volume are characterized by different fringe spacings.
According to the technique described by Rogers, each one of the three sets of fringe plane patterns produced as an aerosol particle passes through the focal volume modulates the intensity of light scattered in the direction of the corresponding one of the three unit vectors. The modulation of the intensity of the scattered light in each direction has a characteristic modulation frequency, whereby a component of the motion of the aerosol particle relative to the aircraft in the direction of any one of the three unit vectors causes an apparent shift in the modulation frequency of the light scattered in that direction. From the shifts in modulation frequency for the three directions defined by the three unit vectors, the magnitude and direction (i.e., the velocity) of the motion of the aerosol particle can be determined.
Fringe pattern techniques developed in the prior art for modulating the intensity of light scattered from atmospheric aerosol particles were not readily adaptable to aircraft velocimetry, because coherent light sources of sufficient power were generally unavailable in the small-scale sizes and compact configurations required for practicable aircraft instrumentation. Furthermore, laser beam sources that were available in the prior art were generally unable to maintain coherent propagation under the extreme operating conditions and harsh environments routinely experienced by high-performance aircraft.